Microsoft has come out swinging after the FCC trashed its prototype "white …

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The FCC issued quite a damning report on "white spaces" devices back on August 1, finding that the most advanced prototype they had been given to test failed miserably. Now Microsoft, the maker of the device, says it knows why: the device was broken. Furthermore, the FCC had a working backup unit in its possession that it never bothered to test.

In a document filed with the FCC yesterday, Microsoft described a meeting that its engineers had with representatives from the FCC's Office of Engineering and Technology on August 9 and 10. In that meeting, the Microsoft reps presented data from their own testing of a white spaces unit and concluded that it "detected DTV signals at a threshold of -114dBm in laboratory bench testing with 100 percent accuracy, performing exactly as expected."

How to explain the difference between Microsoft's tests and the FCC's tests of "Prototype A"? Simple; Prototype A was broken.

Microsoft and FCC engineers agreed that both groups had used proper (and identical) test methodologies, making the Microsoft engineers curious about the poor performance of their prototype unit in FCC testing. While FCC engineers watched, the Microsofties examined Prototype A and found that "the scanner in the device had been damaged and operated at a severely degraded level." This accounted for the unit's general inability to detect when particular channels were occupied.

Prototype A during initial FCC testing

The Microsoft letter also reveals that the FCC had a second copy of Prototype A in its possession the entire time but never bothered to test it. Microsoft engineers tested this backup device in the presence of FCC engineers and found that it performed just as well as the unit they had back at Microsoft. In addition, after talking to FCC engineers, Microsoft was able to tweak the algorithms used to detect wireless microphones. On August 10, the engineers tested the revised device and found that it "detected wireless microphone signals at power levels of -114dBm," the level that Microsoft had been aiming for.

And in the FCC's original tests, the Philips-supplied Protoype B also worked quite well at detecting digital television signals. Taken together, these two results seem to indicate that the white spaces technology works much better than the FCC report led people to believe. So should we prepare for the coming white spaces panacea?

Meet the critics

Not so fast. The powerful National Association of Broadcasters has never been pleased with the idea of using unused TV channels to stream IP data through the air, and Microsoft's response has yet to convince them. Dennis Wharton, executive vice president of media relations at NAB, said in a statement that "the FCC performed rigorous tests on the Microsoft devices, and we are confident that its finding that these devices cause interference to television reception is accurate."

Based on the wording of the statement, it appears that NAB officials didn't bother to actually read Microsoft's filing before trashing it, so take these comments with a grain of salt. "By continuing to press its self-serving agenda," Wharton continued, "Microsoft is playing Russian Roulette with America's access to interference-free TV reception." The NAB seems less interested in disputing the actual science of the test results then in sowing uncertainty about how white spaces devices could impact over-the-air TV reception—an issue with huge political implications.

But even if the devices can operate without causing interference, other technical challenges could limit their usefulness. "Theoretically it makes sense if they can make it work," ABI Research VP Stan Schatt tells Ars. "I'm not convinced they can make it work."

Schatt notes that the devices will use different frequencies in every market in which they are deployed and points out that device makers never know how much white space will be available to any particular unit in any particular location at any particular time. While DTV broadcasts should remain stable, white spaces devices also have to take account of a wireless microphones. A church music minister has already filed a comment with the FCC, in fact, pointing out that thousands of churches across the country use such microphones on a routine basis; disrupting those systems would cost churches millions of dollars.

This makes it difficult for network operators to provide any sort of quality of service guarantees or to offer service level agreements (SLAs) to customers. "If you have QoS requirements, it's very tough to provide SLAs if you're standing in shifting sands and you never know how much white space you'll have at any given time," Schatt says.

Still, assuming that Microsoft's description of what transpired is accurate, there's reason to hope that the technology can become viable after all. Remember that these are still extremely early days for the prototype devices; the Philips unit wasn't even set up to transmit, only to receive. If the devices can in fact reliably detect DTV signals already, the future of white spaces technology looks a bit less bleak than it did two weeks back.